The salts of glyphosate and their preparation have been previously disclosed. The most commonly used salt of glyphosate has been the mono isopropyl ammonium however recently the potassium salt has been favoured by some due to the capacity to include a higher loading of glyphosate as the potassium salt in a given volume of water.
Glyphosate is a broad spectrum herbicide and normally almost all green plants are killed by glyphosate. Great care, therefore, has to be exercise to avoid contact with crops. Recently, however, gene technology has allowed glyphosate resistance to be conferred on crops. This technology has been utilized on a number of important commercial crops including cotton, canola, beet crops, rice, corn and soy bean crops to allow non-discriminatory application to the crop area to be used to selectively control weeds within the crop.
EP-0 218 571 relates to a cloning or expression vector comprising a gene which encodes EPSPS polypeptide which, when expressed in a plant cell, confers glyphosate tolerance to plants. EP-0 293 358 further relates to the enhancement of the effectiveness of glyphosate-tolerant plants by producing mutant EPSP synthase enzymes which exhibit a lower affinity for glyphosate while maintaining catalytic activity. WO 92/00377 discloses genes encoding a glyphosate oxidoreductase enzyme. The genes are useful in producing transformed plants which degrade glyphosate herbicide and are tolerant to glyphosate herbicide. WO 92/04449 discloses genes encoding class II EPSPS enzymes, which are useful in producing transformed plants that are tolerant to glyphosate herbicide.
Glyphosate resistant crops were introduced in the late 1990s and have been widely adopted by growers. As a result, the use of glyphosate resistant crops, particularly cotton such as “Roundup Ready™” cotton, is widespread and generally speaking is favoured by growers due to improved weed control and consequentially improved yields. The symbol “™” indicates a trademark of the Monsanto Company, St Louis, Mo., USA.
Despite the success of glyphosate resistant crops, their susceptibility to glyphosate damage has remained a problem. The rates of glyphosate required for effective weed control in some cases produces plant damage, such as leaf speckling. Also, the use of glyphosate on resistant crops must generally be limited to specific growth stages, such as before the four-leave growth stage in cotton, and certain periods (e.g. 10 days in cotton) must be allowed before respraying. A review of some of the requirements for glyphosate use on glyphosate resistant plants is provided by May et al. in the paper “Transgenic Cotton with Improved Resistance to Glyphosate Herbicide”, Crop Sci. 44:234-240 (2004). These problems lead to further research into genetic modification in an attempt to improve the level of resistance. Recently, “Roundup Ready Flex™” cotton has been introduced, which includes a further genetic modification to extend the growth stages during which glyphosate may be used without crop losses. However, injury may still occur with application of glyphosate composition to “Roundup Ready Flex™” cotton and improvements in the margin between weed control and transgenic crop injury are desirable. We have found that a certain mixture of glyphosate salts exhibit a reduced level of injury to glyphosate resistant crops.